Pressboard and process for its preparation

ABSTRACT

High temperature resistant pressboard having a desirable combination of compression set values and oil absorption is prepared by a process whereby a low density pressboard is first prepared by forming a wet lap of multiple layers of a waterleaf containing 50-95% by weight water and pressing the wet lap at 100-200° C. under a pressure of 10-60 kg/cm 2 , drying, ultimately at 270°-320° C. until substantially no further moisture is evolved and finally pressing at 270°-320° C. under a pressure of 8-350 kg/cm 2  and optionally cooling under restraint.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of application Ser. No. 697,797, filed Feb. 4,1985, abandoned, which is a continuation-in-part of Ser. No. 589,601,filed Mar. 14, 1984, abandoned.

DESCRIPTION

1. Technical Field

This invention relates to an improved aromatic polyamide pressboardhaving increased resistance to compression combined with relatively highoil absorption characteristics. The invention also relates to a processfor preparing the improved pressboard.

2. Background of the Invention

Pressboard prepared from cellulosic materials has been known andcommercially used for many years. While the cellulosic pressboard isextremely useful, its use at high temperature is limited by the lowthermal stability of cellulosic materials.

More recently, aromatic polyamide fibers (U.S. Pat. Nos. 3,063,966 and3,133,138), fibrids (U.S. Pat. No. 2,999,788) and paper (U.S. Pat. No.3,756,908) having excellent properties at high temperatures have becomeknown. Pressboard comprised of aromatic polyamide fibers and fibrids isalso known and can readily be prepared using the same procedures used inthe preparation of cellulosic pressboard.

Aromatic polyamide pressboard has been found to be useful in manyapplications. For example, in oil filled transformers it has been foundto have a suitably high oil absorption which contributes to goodelectrical insulating properties. However, for some uses, it isnecessary that the pressboard not only have a suitably high oilabsorption but also provide resistance to compression so that thepressboard can provide suitable separation of electrically conductingcomponents. It has been found that compaction processes as taught by theprior art either do not provide pressboard products having adequateresistance to compression, or that they do so only by providing apressboard product which does not have adequate oil absorption.

This invention provides an improved aromatic polyamide pressboard havinga combination of good resistance to compression and adequate oilabsorption. This invention also provides a process for the preparationof the improved pressboard.

BRIEF DESCRIPTION OF THE INVENTION

This invention provides a high density pressboard comprised of 20-95% byweight aromatic polyamide fibrids and 80-5% by weight high temperatureresistant floc, said pressboard having a calculated void volume of 13 to28% by volume of the pressboard, a thickness of 0.5 to 50 mm, a mercuryintrusion volume at low surface/volume, V_(ml), of less than 0.20 cm³/g; a mercury intrusion volume at high surface/volume, V_(mh), of 0.08to 0.28 cm³ /g, an oil absorption by volume in cm³ /g, V_(o), of 0.09 to0.28 and by weight 8-24%; and a total available absorption volume in cm³/g, V_(a), equal to the largest of the values for V_(ml), V_(mh), andV_(o) ; the ratio of V_(a) to V_(ml) being at least 1.1; said pressboardhaving a compression set (as hereinafter defined) of greater than 0.12mm but no more than 0.5 mm. Preferably the pressboard is comprised of50-70% by weight aromatic polyamide fibrids and 30-50% by weight hightemperature resistant floc. Preferably the high temperature resistantfloc consists of an aromatic polyamide and the pressboard has a densityof 1.00 to 1.20 g/cm³. Preferably the aromatic polyamide fibrids andhigh temperature resistant floc consist essentially of poly(m-phenyleneisophthalamide)(MPD-I). The pressboard preferably is comprised ofaromatic polyamide fibrids and floc and has a thickness of 1 to 10 mm, adensity of 1.02 to 1.17 g/cm³, most preferably 1.10 to 1.15 g/cm³. Thepressboard preferably has a compression set of 0.12 to 0.35 mm, mostpreferably 0.20 to 0.30 mm.

The improved pressboard is prepared by a process whereby an aqueousslurry having 0.1 to 2% by weight total solids comprised of 20-95% byweight aromatic polyamide fibrids and 80-5% by weight high temperatureresistant floc having a length of 2 to 12 mm., said aromatic polyamidefibrids and high temperature resistant floc having a melting pointhigher than 320° C., the slurry is formed into a waterleaf having awater content of 50-95% by weight, the waterleaf is combined intomultiple layers to form a wet lap, the wet lap is pressed at 100° to200° C. under a pressure of 10 to 60 kg/cm² to form a low densitypressboard having a calculated void volume of 30 to 60% by volume of thepressboard, the low density pressboard is dried, ultimately at 270°-320°C., until substantially no further moisture is evolved and finallypressed at 8 to 350 kg/cm² at 270°-320° C. Preferably the temperature is275°-300° C. Most preferably, the final pressing is at 275°-285° C. andthe pressure is 15 to 70 kg/cm². Preferably the pressboard is cooledunder restraint. Preferably the high temperature resistant floc consistsof an aromatic polyamide. Preferably the aromatic polyamide fibrids andthe high temperature resistant floc consist of poly(m-phenyleneisophthalamide).

DETAILED DESCRIPTION OF THE INVENTION Definitions

By "aromatic polyamide" is meant nonfusible polyamides wherein the amidegroup, i.e., the ##STR1## radical where R is hydrogen or a 1-6 carbonalkyl group, of each repeating unit is linked through the nitrogen atomand the carbon atom to a carbon atom in the ring of separate aromaticring radicals. The term "aromatic ring" is defined herein as acarbocyclic ring possessing resonance.

By "aromatic polyamide fibrids" is meant small, nongranular, nonrigidfibrous or film-like particles of an aromatic polyamide having a meltingpoint higher than 320° C. Two of their three dimensions are of the orderof microns. Their smallness and suppleness allows them to be depositedin physically entwined configurations such as are commonly found inpapers made from wood pulps. Fibrids can be prepared by precipitating asolution of the aromatic polyamide into a coagulant such as in apparatusof the type disclosed in U.S. Pat. No. 3,018,091.

By "high temperature resistant floc" is meant short fibers, typicallyhaving a length of 2 to 12 mm and a linear density of 1-10 decitex, madeof a material having a melting point higher than 320° C., such asaromatic polyamides, aromatic polyamide-imides, aromatic polyimides,polybenzimidazoles, etc., or inorganic materials such as glass, ceramicmaterials, alumina, etc. Other high temperature resistant materials suchas mica may also be present in relatively finely subdivided form.

By "aromatic polyamide floc" is meant short fibers cut from fibersprepared by the processes described in U.S. Pat. Nos. 3,063,966,3,133,138, 3,767,756, and 3,869,430.

Conventional aromatic polyamide pressboard may be prepared by feeding anaqueous slurry of MPD-I fibrids and MPD-I floc to a cylinder paperforming machine whereby water is removed and multiple layers of fibrousmaterial having a water content of 50-95% by weight of the wet sheet isbuilt up to a wet lap of the desired thickness. The wet lap is cut fromthe cylinder, laid flat and pressed at 100°-200° C., under a pressure of10-60 kg/cm². The resulting conventional pressboard usually has a highoil absorption of 20-50% by weight, a density of about 0.7 to 0.9 g/cm³,a calculated void volume of about 35 to 50% by volume of the pressboard,mercury intrusion volume of about 0.30 to 0.50 cm³ /g, both at low andhigh surface/volume, a ratio of total available absorption volume in cm³/g, V_(a), to the mercury intrusion volume at low surface/volume,V_(ml), of about 1 and a compression set of 0.75 to 2.5 mm.

However, for some uses, such as spacers used in oil filled transformers,the compression set desirably should be not less than about 0.12 mm ormore than about 0.5 mm while maintaining an oil absorption of at least8%. Pressboard with compression set values of less than about 0.12 mm donot have the combination of compressibility and resilience necessary tomaintain proper spacing of electrical components in, e.g., transformers.Pressboard with compression set values greater than 0.5 mm likewise donot maintain proper spacing of components.

The above desired properties are provided by the product of thisinvention. It has been found that when a low density pressboard having acalculated void volume of 30 to 60% prepared as described above isfurther dried, ultimately at a temperature of 270°-320° C., untilsubstantially no further moisture is evolved and then pressed at270°-320° C. and a pressure of 8 to 350 kg/cm², preferably followed bycooling under restraint, a pressboard having the desired properties isobtained. The drying is preferably accomplished by step-wise increase intemperature. Moisture evolution is facilitated by application andrelease of light pressure. In general, the pressing is preferably at275°-300° C. at 15 to 70 kg/cm² for at least 5 minutes but thickproducts may require pressing for longer times. More than one layer oflow density pressboard may be combined during high temperature pressing.In this case, longer pressing times should be employed. Preferably thehigh temperature pressing should be above the glass transitiontemperature (T_(g)) of the aromatic polyamide comprising the fibridswhich in the case of the preferred poly(m-phenylene isophthalamide)fibrids is about 275° C.

It has been found that the process described above, wherein a wet lap isformed of multiple layers of waterleaves having a water content of50-95% and the wet lap is pressed at 100°-200° C. under a pressure of10-60 kg/cm² to prepare a low density pressboard having a calculatedvoid volume of 30-60%, and the low density pressboard is then dried andpressed again at 270°-320° C. under a pressure of 8-350 kg/cm², isessential for obtaining a pressboard product exhibiting good resistanceto compression as well as adequate oil absorption. If the low densitypressboard has a calculated void volume of less than 30%, the oilabsorption of the final pressboard product tends to be very poor.

The pressboard of this invention is useful in clamping rings and inaxial and radial spacers in oil filled electrical transformers.

Products of this invention have a calculated void volume of 13 to 28% byvolume of the pressboard, mercury intrusion volumes at lowsurface/volume, V_(ml), of less than 0.20 cm³ /g and at highsurface/volume, V_(mh), of 0.08 to 0.28 cm³ /g, an oil absorption byvolume in cm³ /g, V_(o), of 0.08 to 0.28 and by weight of 8-24% and atotal available absorption volume in cm³ /g, V_(a), equal to the largestof the values for V_(ml), V_(mh) and V_(o), the ratio of V_(a) to V_(ml)being at least 1.1. Pressboard having a calculated void volume of morethan 28% or a ratio of V_(a) to V_(ml) of 1.0 generally exhibits poorcompression set, while pressboard having a calculated void volume ofless than 13% or V_(mh) less than 0.08 cm³ /g generally exhibits pooroil absorption. The products of this invention have V_(a) values whichare quite different from V_(ml) values, the ratio of these being atleast 1.1 and as high as 4.

TESTS

Density. Dry pressboard is cut into a rectangular sample measuring atleast 10 cm×10 cm (4 in×4 in), preferably at least 20 cm×20 cm (8 in×8in), making sure that the corners are cut square so that the upper andlower faces of the sample are of the same area and that the dimensionscan be measured accurately. The length and width of the rectangularsample are measured to an accuracy of at least 0.25 cm (0.1 in). Thethickness of the rectangular sample of pressboard is measured in atleast ten places spaced substantially equally apart around all sides ofthe pressboard, away from the edges, using a micrometer caliper whichcontacts the sample with surfaces having a diameter of about 0.6 cm(0.25 in) at a pressure of about 0.1 kg/cm² (about 1.25 psi), to anaccuracy of at least 0.00025 cm (0.1 mil), averaging the ten thicknessmeasurements. The sample of pressboard is then weighed to the nearest0.0001 g. The volume of the sample of pressboard V_(b) is thencalculated in cm³ and the weight is divided by the volume to give thedensity in g/cm³.

Calculated Void Volume. The void volume in cm³, V_(v), of a sample ofthe pressboard is determined from the relationship

    V.sub.b =V.sub.m +V.sub.v,

or

    V.sub.v =V.sub.b -V.sub.m,

where

V_(b) is the volume of the pressboard in cm³ as determined above, V_(m)is the total volume in cm³ of all the materials comprising thepressboard, and V_(v) is the remaining volume in cm³, which is taken asthe void volume. V_(m) is determined from the weights and densities ofeach of the materials of which the pressboard sample is made, calculatedas follows: ##EQU1## where W_(f) is the weight in g of the aromaticpolyamide fibrids in the pressboard sample, W_(i) is the weight in g ofthe floc (including any other non-fibrid high temperature resistantmaterial) in the pressboard sample, and ρ_(i) is the density of thematerial of which the floc is made [1.38 g/cm³ for MPD-I and 1.44 g/cm³for poly(p-phenylene terephthalamide)]. When there is more than one kindof floc (or other high temperature resistant material such as mica),W_(i) /ρ_(i) is calculated as follows: ##EQU2## where i=1, . . . , n.The calculated void volume as a percentage volume, % V_(v), is thencalculated as follows: ##EQU3## In the case of a 100% MPD-I pressboardsample having a weight in g of W_(b) and a volume in cm³ of V_(b), andsince for this case ##EQU4## the equation reduces to: ##EQU5##

The calculated void volume is a measure of all of the voids, bothisolated voids and interconnected voids, in a sample of pressboard.

Oil Absorption. This test is carried out in accordance with the methoddescribed by the International Electrotechnical Commission, IECStandard, Publication 641-2, First edition (1979), "Specification forpressboard and presspaper for electrical purposes, Part 2: Methods oftest," pages 29 and 31 (section 17), published by Bureau Central de laCommission Electrotechnique Internationale Geneva, Switzerland. Theresult is expressed to the nearest 0.1% as a percentage by weight oilabsorption on the original mass of the pressboard sample tested. The oilabsorption by volume in cm³ /g, V_(o), is then calculated by dividingthe percentage by weight oil absorption by the density of the sample ofpressboard. V_(o) values are initially reported to the same number ofsignificant figures as the percentage by weight oil absorption, thenrounded to two decimal places.

Compression Set. The pressboard to be tested is cut into rectangularstrips 3.8 cm (1.5 in) wide×5.1 cm (2.0 in) long and a sufficient numberof the strips are stacked to make a stack approximately 5.1 cm (2.0 in)high. The stack of samples is placed in an oven for 48 hrs. at 110° C.,then taken from the oven and placed in a conventional machine fortesting compressive properties, equipped for constant rate of crossheadmovement and having a capacity of at least 10,000 kg (22,000 lb.) (e.g.,the Tinius Olsen Universal Testing Machine, Model 60 SDT,Servo-controlled, 60,000 lb. capacity, Super L UTM, made by the TiniusOlsen Universal Testing Machine Co., Inc., Easton Rd., Willow Grove, PA19090 equipped with a Model MM Flat Bed X-Y Recorder manufactured byHouston Instruments, Inc. and Tinius Olsen Model D-2 and D-4Deflectomers for accurately measuring the deflection of compressedsamples at two different chart magnifications). In carrying out thetest, the load is applied at the constant rate of 0.5 cm per min. (0.2in. per min.) and released. A load of 680 kg (1,500 lb.), equivalent to35 kg/cm² (3,448 kPa; 500 psi), is applied to the stack of samples, andthe load is then immediately released to a load of 136 kg (300 lbs.).This load, equivalent to 7 kg/cm² (690 kPa; 100 psi), is designated asthe bedding pressure, and the load is released to this bedding pressurebetween each cycle. The stack of samples is next cycled to 1361 kg(3,000 lbs.), equivalent to 70 kg/cm² (6,895 kPa; 1,000 psi), returningto the bedding pressure. It is then cycled to 2,722 kg (6,000 lbs.),equivalent to 141 kg/cm² (13,790 kPa; 2,000 psi), returning to thebedding pressure. Finally it is cycled to 4,082 kg (9,000 lbs.),equivalent to 211 kg/cm² (20,685 kPa; 3,000 psi), and back once more tothe bedding pressure. The compression set is taken as the loss in heightin mm (alternatively in mils) of the stack of samples, as measured bythe deflectometer, upon the return to the bedding pressure after thefinal cycle. It is preferred to have the deflectomer readingscontinuously plotted on a chart so that the entire sequence of cycles isdisplayed on a graph for each sample tested.

It the amount of sample material is limited, the 3.8 cm×5.1 cmrectangular strips are stacked to a lesser height, preferably at least2.55 cm (1.0 in.) high, and the deflection after the final cycle ismultiplied by the appropriate factor to scale the result to correspondto the result which would be obtained from a stack 5.1 cm (2.0 in.)high.

Mercury Intrusion Volume. In this determination a conventional mercuryporosimeter (Aminco Mercury 60,000 psig max, Newport Scientific Co.,Inc., Silver Spring, MD 20910) is employed to determine the volume ofmercury which can be forced into the pores, or interconnected voids, ofa porous sample. To determine whether the surface area of a given weightof the pressboard has an effect on the volume of mercury which can beforced into its pores, determinations are made both on lowsurface/volume samples and high surface/volume (subdivided) samples ofthe pressboard.

The nominal weight of each sample tested is 0.3 g. To prepare the lowsurface/volume and high surface/volume samples, an initial sampleslightly heavier than 0.6 g and preferably rectangular in shape is cutfrom the pressboard to be tested. The initial sample is then cut down insize (e.g. with a pair of side-cutters) in a series of approximately 25to 35 clean cuts straight through the pressboard near its edges toproduce a corresponding number of fragments, leaving a preferablyquadrilateral sample weighing about 0.3 g which is taken as the samplefor the low surface/volume measurement. This low surface/volume sampleshould be of such shape that it will fit intact in the penetrometer bulb(sample chamber) of the porosimeter, if at all possible. If the sampleis very thin and a single piece weighing 0.3 g which will fit in thepenetrometer bulb cannot be prepared, the low surface/volume sample isprepared in the form of two or even three pieces which will fit in thebulb. The low surface/volume sample is weighed to the nearest 0.0001 gon glassine paper. A sufficient number of the pressboard fragments,preferably about 25 to 30, to weigh about 0.3 g are placed on glassinepaper (preferably they are collected on the glassine paper as they arecut) as the high surface/volume (subdivided) sample. The subdividedsample is weighed to the nearest 0.0001 g.

To conduct the determination, a weighed sample is placed in the openpenetrometer bulb, after which the bulb is capped and evacuated untilthe vacuum gauge displays a pressure of 50 microns of mercury or less.The filling device is then tilted backward until its stop is reached, sothat the tip of the penetrometer is immersed in mercury. The stopcock onthe filling device is gradually opened to admit air to the systemslowly, causing mercury to enter the penetrometer bulb, tapping thetubes to aid in wetting the sample with mercury. After total wetting hasbeen achieved, the filling device is returned to vertical position. Thepenetrometer is then moved from the vacuum chamber to the pressurechamber.

The pressure is then gradually increased, recording penetrometerreadings at intervals as the pressure increases. The equipment iscustomarily provided with more than one pressure gauge, e.g. recordingmaximum values of about 350 kg/cm² (34 MPa; 5,000 psi) and about 4200kg/cm² (414 MPa; 60,000 psi), and if so the equipment is switched overto the high pressure gauge at the appropriate time as the pressureincreases. The penetrometer reading at 4200 kg/cm² (414 MPa; 60,000 psi)is recorded at the conclusion of the test. The mercury intrusion volumeat 4200 kg/cm² is determined from the penetrometer reading in accordancewith the instructions provided by the manufacturer of the equipment. Fora particular specimen of pressboard, mercury intrusion volume values incm³ /g (cm³ of mercury at 4200 kg/cm² pressure per g of pressboard) arefirst determined to four decimal places, then rounded and finallyreported to two decimal places both for the low surface/volume and highsurface/volume (subdivided) samples. If desired, graphs of mercuryintrusion volume values over the entire pressure range are constructed,based on the penetrometer readings taken at intervals throughout thetest. The mercury intrusion volume at low surface/volume is designatedby the symbol, V_(ml), and the mercury intrusion volume at highsurface/volume is designated by the symbol, V_(mh).

Total Available Absorption Volume. The total available absorptionvolume, V_(a), of a pressboard sample is taken as being equal to thelargest of the values for V_(ml), V_(mh) and V_(o) (all values prior torounding) for the sample. For any given sample of pressboard, V_(a) is ameasure of the volume in cm³ per g of the interconnected voids in thesample which are accessible to penetration by liquids.

The ratio, V_(a) /V_(ml), is then calculated, using values of V_(a) andV_(ml) prior to rounding in making the calculation. In reporting theratio, it is rounded to one decimal place. A value of this ratio equalto or greater than 1.1 is indicative of a structure of limited orpartial accessibility of internal voids in the pressboard, a structureassociated with good compression resistance of the pressboard when thecalculated void volume of the pressboard is no more than 28%.

EXAMPLE 1

A. Preparation of "Standard Pressboard"

Filaments of poly(m-phenylene isophthalamide) (MPD-I) having an inherentviscosity of 1.5 were dry spun from a solution containing 19% MPD-I, 70%dimethylacetamide (DMAc), 9% calcium chloride, and 2% water. On leavingthe drying tower the as-spun filaments were given a preliminary washwith water so that they contained about 60% DMAc, 15% calcium chloride,and 100-150% water, based on the weight of dry polymer. The filamentswere washed and drawn 4X at 90° C. in a counter-current extraction-drawprocess in which the calcium chloride determined as chloride content andDMAc content were reduced to about 0.1% and 0.5%, respectively. Thefilaments were crystallized immediately after drawing by passing themover hot rolls at a temperature of about 340° C. The filaments soproduced had a linear density of 2.2 decitex (2.0 denier), a tenacity ofabout 3.7 dN/tex (4.2 g/denier), an initial modulus of 70 dN/tex (79gpd) and an elongation of 34%. The filaments were cut to floc having alength of 3.4 mm (0.135 in).

Fibrids of MPD-I having an inherent viscosity of 1.5 were preparedsubstantially as described by Gross in U.S. Pat. No. 3,756,908, issuedSept. 4, 1973, column 5 lines 34-54, stopping short of the refiningstep.

An aqueous slurry was prepared containing 1.0 wt. % fibrids and flochaving a composition of 60% of the above MPD-I fibrids and 40% of theabove MPD-I floc. The slurry was held in an agitated vessel and thenpumped to a double disc refiner (Beloit Jones Model 3000 20-inch DoubleDisc refiner, made by the Jones Division of the Beloit Corporation,Dalton, Mass. 01226), equipped with refining discs containing narrowbars and channels with surface dams. The plates of the refiner werepositioned with a gap of 0.5 mm (20 mils) between the rotor and thestator plates. The rotor plates were operated at 900 rpm. After passingthrough the refiner, the slurry was passed through a second refinerunder the same operating conditions. After the two passes through therefiners the fibrids in the slurry were well reduced in size and wellopened into fibrid films, while the floc fibers were well distributedamong the fibrids. The slurry made in this way was then diluted toapproximately 0.1% by weight solids and fed to a conventional cylinderwet paper-making machine upon which a continuous sheet of wet paper wasmade and transferred to an endless felt, the moisture content beingadjusted by suction and pressure to about 400% based on solids (80% byweight based on the wet sheet). The weight of the solids in the wetpaper was approximately 36 g/m². The continuous wet sheet was nextdelivered to a forming roll, where it was wound continuously on acylindrical tube until it overlapped about 70 times. A longitudinal cutwas then made in the layered paper and the entire thickness of wet lap(wet layered paper) was then removed and placed between the platens of ahot press, the platens being maintained at 140° C. and having beencovered with wire screen to facilitate moisture removal. The press wasloaded at contact pressure, and the pressure was then raised to andmaintained for one hour at 35 kg/cm² (3450 kPa; 500 psi) while theplatens of the press were maintained at 140° C. The product, hereindesignated as "Standard Pressboard", was a low density aramid pressboardapproximately 3.2 mm (126 mils) thick. It was found to have a density of0.82 g/cm³, a calculated void volume, % V_(v), of 41% by volume of thepressboard, a compression set of 2.13 mm (84 mils), and an oilabsorption of 32.5%. V_(o) was 0.38 cm³ /g, V_(ml) was 0.38 cm³ /g(rounded from 0.3791), and V_(mh) was 0.42 cm³ /g (rounded from 0.4197).V_(a) for this Standard Pressboard sample was 0.42 cm³ /g and the ratioV_(a) /V_(ml) was 1.1.

B. Preparation of Compression-Resistant Pressboard

A 30.5 cm×30.5 cm (12 in×12 in) square sheet of the "StandardPressboard" prepared as in Part A above was predried at 150° C. for atleast 2 hours and then placed between the platens of a flat press(Machine No. 9175-M, Watson Stillman Press Division, Farrel Company,Emhart Machinery Group, 25 Main St., Ansonia, Conn. 06401). With theplatens preheated to 280° C. and maintained at that temperature, apressure of 19.5 kg/cm² (1910 kPa; 277 psi) was applied to the "StandardPressboard" for a total of 20 minutes, releasing the pressure for a fewseconds and then reapplying it after a total of 1, 2, 3, 6, 12, and 16minutes to permit escape of any trapped gases. After a total of 20minutes of hot pressing, the pressboard was taken out hot, placed inanother press at room temperature, and allowed to cool under a pressureof 2.8 kg/cm² (276 kPa; 40 psi), just sufficient to keep the pressboardflat while cooling. The product, designated as "Sample 1A", was anaramid pressboard approximately 2.45 mm (96.5 mils) thick (thicknessrange 2.35-2.53 mm). It was found to have a density of 1.11 g/cm³, a %V_(v) of 20%, a compression set of 0.30 mm (12 mils), and an oilabsorption of 12.71%. V_(o) was 0.15 cm³ /g, V_(ml) was 0.15 cm³ /g(rounded from 0.1502), and V_(mh) was 0.17 cm³ /g (rounded from 0.1700).V_(a) for Sample 1A was 0.17 cm³ /g and the ratio V_(a) /V_(ml) was 1.1.

Another sheet of predried "Standard Pressboard" was subjected to thesame procedure, except that a pressure of 18.5 kg/cm² (1813 kPa; 263psi) was applied in the press for a total of 20 minutes at 280° C. Theproduct, designated as "Sample 1B", was approximately 2.5 mm (98.7 mils)thick (thickness range 2.38-2.60 mm). It was found to have a density of1.08 g/cm³, a % V_(v) of 22%, a compression set of 0.36 mm (14 mils),and an oil absorption of 12.19%. V_(o) was 0.14 cm³ /g, V_(ml) was 0.16cm³ /g (rounded from 0.1551), and V_(mh) was 0.17 cm³ /g (rounded from0.1743). V_(a) for Sample 1B was 0.17 cm³ /g and the ratio V_(a) /V_(ml)was 1.1.

EXAMPLE 2

Two 46 cm×122 cm (18 in×48 in) rectangular sheets of "StandardPressboard", prepared substantially as described in Part A of Example 1but having a thickness of 3.0 mm (118 mils), were aligned in a stackabove and below a 46 cm×122 cm sheet of 1.6-mm thick "StandardPressboard", similarly prepared except that proportionately feweroverlaps of wet paper were wound on the cylindrical roll in the formingstep. All of the sheets were predried at 150° C. just before forming thestack. The aligned stack was then placed immediately in a hot presshaving platens oil-heated to 280° C. (535° F.) and subjected to three2-minute cycles of contact pressure (3.5 kg/cm²) at 280° C. followed bythe release of pressure. A one-minute cycle of pressure at 28 kg/cm²(2758 kPa; 400 psi) and quick release was followed by a one-minute cycleof pressure at 35 kg/cm² and quick release, after which pressure wasapplied at 35 kg/cm² for fifteen minutes while the platens weremaintained at 280° C. The pressboard product was taken out hot andplaced under contact pressure in a separate press, initially at roomtemperature and water-cooled to absorb the heat of the pressboard, tokeep it flat while cooling. The product, designated "Sample 2", was anaramid pressboard approximately 5.3 mm thick (210 mils). It was found tohave a density of 1.12 g/cm³, a % V_(v) of 19%, a compression set of0.13 mm (5 mils), and an oil absorption of 9.3%. V_(o) was 0.11 cm³ /g,V_(ml) was 0.09 cm³ /g (rounded from 0.0940), and V_(mh) was 0.17 cm³ /g(rounded from 0.1665). V_(a) for Sample 2 was 0.17 cm³ /g and the ratioV_(a) /V_(ml) was 1.8.

EXAMPLE 3

A 46 cm×81 cm (18 in×32 in) rectangular sheet of 2.1-mm thickpressboard, prepared substantially like the "Standard Pressboard" ofPart A of Example I, except that proportionately fewer overlaps of wetpaper were used, was placed without predrying in a press equipped forelectrical heating and water cooling. Initially the press was at 66° C.(150° F.) and contact pressure, about 3.5 kg/cm² (345 kPa; 50 psi). Thepress was heated over about 20 minutes under the same contact pressure,with no intervals of pressure release, to about 280° C. (about 353° F.).The pressure was then increased to 35 kg/cm² (3448 kPa; 500 psi) andmaintained at that pressure, with no release of pressure, for 12 minuteswhile the press was maintained at 280° C. The electrical heating wasthen discontinued and the press was then cooled back down to 66° C. withcirculation of cool water over a 20-minute period while the pressure wasmaintained at 35 kg/cm². The product, designated "Sample 3", was anaramid pressboard approximately 1.6 mm thick (64 mils). It was found tohave a density of 1.13 g/cm³, a % V_(v) of 18%, a compression set of0.13 mm (5 mils), and an oil absorption of 9.32%. V_(o) was 0.11 cm³ /g,V_(ml) was 0.06 cm³ /g (rounded from 0.0553), and V_(mh) was 0.14 cm³ /g(rounded from 0.1390). V_(a) for Sample 3 was 0.14 cm³ /g and the ratioV_(a) /V_(ml) was 2.5.

EXAMPLE 4

Square sheets of low density pressboard were prepared in substantiallythe same manner as the "Standard Pressboard" of Part A of Example 1,with the following exceptions. Fibrids were refined and mixed with flocat the paper-making machine. Fifty sheets of wet paper were combinedinto a wet lap and the entire wet lap was cut into 20-cm (8-in) squares.In pressing the squares of wet lap at 140° C. under a pressure of 35kg/cm², the pressure was applied for 30 minutes rather than one hour.The low density pressboard so formed was 2.1 mm (81 mils) thick and hada density of 0.88 g/cm³. Its % V_(v) was 36%. The low density pressboardwas predried at 120° C. in an oven for four hours and then placedbetween the platens of a flat press preheated to 280° C. as in Part B ofExample I. Low pressure was applied briefly at first, with three cyclesof release of the pressure to permit escape of the trapped gassesfollowed by reapplication of the pressure. A pressure of 53 kg/cm² (5171kPa; 750 psi) was then applied for a contact time of 1 minute, the hotpressboard finally being cooled under restraint in a separate press. Theproduct, designated as "Sample 4A", was an aramid pressboard 1.75 mm (69mils) thick and had a density of 1.04 g/cm³. Its % V_(v) was 25%. V_(ml)was 0.15 cm3/g (rounded from 0.1542) and V_(mh) was 0.17 cm³ /g (roundedfrom 0.1712), The compression set was 0.20 mm (8.0 mils), the oilabsorption was 15.9% by weight of pressboard, and V_(o) was 0.19 cm³ /g.For Sample 4A, V_(a) was 0.19 cm³ /g and the ratio V_(a) /V_(ml) was1.2.

Other 20-cm square sheets of low density pressboard were prepared in thesame manner, except that the weight of the solids in the wet paper wasapproximately 60 g/m², the wet lap was formed from thirty sheets of wetpaper, and in the pressing step the pressure was applied for 45 minutesrather than 30 minutes. The low density pressboard was 2.1 mm (84 mils)thick and had a density of 0.92 g/cm³. Its % V_(v) was 33%. The lowdensity pressboard was predried and hot pressed in the same manner asSample 4A, except that the pressure of 53 kg/cm² was applied for acontact time of 10 minutes. The product, designated as Sample "4B", wasan aramid pressboard 1.8 mm (71 mils) thick and had a density of 1.15g/cm³. Its % V_(v) was 17%. V_(ml) was 0.05 cm³ /g (rounded from 0.0486)and V_(mh) was 0.15 cm³ /g (rounded from 0.1452). The compression setwas 0.147 mm (5.8 mils), the oil absorption was 9.7% by weight ofpressboard, and V_(o) was 0.11 cm³ /g. For Sample 4B, V_(a) was 0.15 cm³/g and the ratio V_(a) /V_(ml) was 3.0.

EXAMPLE 5

In a series of experiments, low density pressboards were made whichcontained varying ratios of MPD-I fibrids and floc. The low densitypressboards were made in substantially the same manner as the "StandardPressboard" of Part A of Example 1 with the following exceptions.Fibrids were refined and then mixed with flock at the paper-makingmachine in the proportions given below. Wet paper was produced with theweight of the solids being approximately 60 g/m². Thirty sheets of wetpaper were combined into a wet lap which was cut into 20-cm (8-in.)squares and pressed.

In one experiment a mixture of 80% fibrids and 20% floc having a cutlength of 0.32 cm (0.125 in) was used. The low density pressboard wasprepared under a pressure of 17.6 kg/cm² (1724 kPa; 250 psi) for 1.5 hr.at a temperature of 140° C. It was approximately 2.1 mm (82 mils) thickand had a density of 0.92 g/cm³. % V_(v) was 33%.

The low density pressboard was dried at 120° C. for 4 hours and pressedat 280° C. under low pressure at first with brief cycles of release andreapplication of pressure, then for 10 min. at 8.8 kg/cm² (862 kPa; 125psi.) The product, designated "Sample 5A", was an aramid pressboardapproximately 1.9 mm (73 mils) thick, had a density of 1.04 g/cm³, acompression set of 0.21 mm (8.3 mils), an oil absorption of 13.7% byweight pressboard, and V_(o) was 0.16 cm³ /g. % V_(v) for sample 5A wascalculated as 25%. V_(ml) was 0.10 cm³ /g (rounded from 0.0990), V_(mh)was also 0.10 cm³ /g (rounded from 0.0996). For sample 5A V_(a) was 0.16cm³ /g (the largest of the values for V_(ml), V_(o), and V_(mh)) and theratio V_(a) /V_(ml) was 1.6.

The procedure for preparing the low density pressboard was repeated,except that a mixture of 40% fibrids and 60% floc was used and that thepressure applied was 35 kg/cm² (3450 kPa; 500 psi) for a period of 45minutes at 140° C. The low density pressboard was approximately 2.6 mm(103 mils) thick and had a density of 0.78 g/cm³. % V_(v) was 43%. Thelow density pressboard was dried at 120° C. for 4 hours and pressed at280° C., under low pressure at first with brief cycles of release andreapplication of pressure, then for 10 min. at a pressure of 53 kg/cm²(5171 kPa), finally being cooled under restraint in a separate press.The product, designated "Sample 5B", was an aramid pressboardapproximately 2.0 mm (79 mils) thick, had a density of 1.02 g/cm³, acompression set of 0.15 mm (6.0 mils), an oil absorption of 17.1% byweight of pressboard, and V_(o) was 0.20 cm³ /g. % V_(v) was 26%. V_(ml)was 0.17 cm³ /g (rounded from 0.1683) and V_(mh) was 0.27 cm³ /g(rounded from 0.2673). For Sample 5B, V_(a) was 0.27 cm³ /g and theratio V_(a) /V_(ml) was 1.6.

The procedure for preparing the low density pressboard was repeatedagain, except that a mixture of 20% fibrids and 80% floc was used andthat the pressure applied was 35 kg/cm² for a period of 45 minutes at140° C. The low density pressboard was approximately 3.1 mm (123 mils)thick and had a density of 0.70 g/cm³. % V_(v) for this low densitypressboard was 49%. The low density pressboard was dried as describedabove and pressed at 280° C., under low pressure at first with briefcycles of release and reapplication of pressure, then for 10 min. at 79kg/cm² (7763 kPa; 1125 psi). The product, designated "Sample 5C", was anaramid pressboard approximately 2.1 mm (84 mils) thick, had a density of1.03 g/cm³, a compression set of 0.35 mm (13.6 mils), an oil absorptionof 12.1% by weight of pressboard, and V_(o) was 0.14 cm³ /g. % V_(v) was25%. V_(ml) was 0.16 cm³ /g (rounded from 0.1565) and V_(mh) was 0.23cm³ /g (rounded from 0.2342). For Sample 5C, V_(a) was 0.23 cm³ /g andthe ratio V_(a) /V_(ml) was 1.5.

The procedure for preparing the low density pressboard was repeated oncemore, except that a mixture of 95% fibrids and 5% floc was used and thatthe pressure applied was 17.6 kg/cm² for a period of 1.5 hr at 140° C.The low density pressboard was approximately 1.9 mm (75 mils) thick andhad a density of 0.90 g/cm³. % V_(v) was 35%. The low density pressboardwas dried as described above and pressed at 280° C., under low pressureat first with brief cycles of release and reapplication of pressure,then for 10 min at 8.8 kg/cm². The product, designated "Sample 5D", wasan aramid pressboard approximately 1.7 mm (68 mils) thick, had a densityof 1.06 g/cm³, a compression set of 0.34 mm (13.4 mils), an oilabsorption of 11.7% by weight of pressboard, and V_(o) was 0.14 cm³ /g.% V_(v) was 23%. V_(ml) was 0.05 cm³ /g (rounded from 0.0459) and V_(mh)was 0.08 cm³ /g (rounded from 0.0805). For Sample 5D, V_(a) was 0.14 cm³/g and the ratio V_(a) /V_(ml) was 3.0.

EXAMPLE 6

In a series of experiments, low density pressboards based partly onhigh-temperature resistant flocs other than MPD-I flocs were made. Thelow density pressboards were made in substantially the same manner asthe "Standard Pressboard" of Part A of non-MPD-I floc was blended withMPD-I floc having a cut length of 0.32 cm (0.125 in.) and the blend offlocs was mixed at the paper machine with refined fibrids in theproportions given below. Wet paper was produced with the weight of thesolids being approximately 60 g/m². Thirty sheets of wet paper werecombined into a wet lap which was cut into 20-cm (8-in.) squares andpressed under the conditions given in Ex. 1, Part A.

In one experiment, a mixture of 60% MPD-I fibrids, 20% MPD-I floc, and20% commercially available poly(p-phenylene terephthalamide) (PPD-T)floc having a linear density of 1.67 decitex (1.5 denier) and a cutlength of 0.32 cm (0.125 in.) was used to prepare an low densitypressboard having a thickness of 2.9 mm (113 mils), a density of 0.83cm³ /g, and a % V_(v) of 41%. The low density pressboard was dried at120° C. for 4 hours and pressed at 280° C., under low pressure at firstwith brief cycles of release and reapplication of pressure, then at 53kg/cm² (5171 kPa; 750 psi) for 10 min., the hot pressboard finally beingcooled under restraint in a separate press. The product, designated as"Sample 6A" was a pressboard 2.2 mm (86 mils) thick having a density of1.10 g/cm³, a % V_(v) of 22%, a compression set of 0.27 mm (10.8 mils),an oil absorption of 10.8% by weight of pressboard, and a V_(o) of 0.13cm³ /g. V_(ml) was 0.08 cm³ /g (rounded from 0.0787) and V_(mh) was 0.12cm³ /g (rounded from 0.1151). V_(a) for Sample 6A was 0.13 cm³ /g andthe ratio V_(a) /V_(ml) was 1.6.

In another experiment, a mixture of 60% MPD-I fibrids, 35% MPD-I floc,and 5% E-glass fiber floc having a density of 2.4 g/cm³ a linear densityof 3.3 decitex (3 denier), and a cut length of 0.64 cm (0.25 in) wasused to prepare an low density pressboard having a thickness of 2.2 mm(88 mils), a density of 0.91 cm³ /g, and a % V_(v) of 36%. The lowdensity pressboard was dried and pressed by the same procedure describedabove for making Sample 6A. The product, designated as "Sample 6B", wasan aramid/glass fiber pressboard 1.8 mm. (71 mils) thick having adensity of 1.15 g/cm³, a % V_(v) of 20%, a compression set of 0.18 mm (7mils), an oil absorption of 8.6% by weight of pressboard and a V_(o) of0.10 cm³ /g. V_(ml) was 0.06 cm³ /g (rounded from 0.0576) and V_(mh) was0.14 cm³ /g (rounded from 0.1424). V_(a) for Sample 6B was 0.14 cm³ /gand the ratio V_(a) /V_(ml) was 2.5.

CONTROL SAMPLES OUTSIDE THE INVENTION

(1) The procedure of Example 1, Part B, was repeated, except that thepressure was increased to 53 kg/cm² (5171 kPa; 750 psi), the press againbeing maintained at a temperature of 280° C. This product, designated as"Control 1" had an oil absorption of only 2.03%. It was about 2.2 mm (87mils) thick (thickness range 2.14-2.31 mm) and had a density of 1.21g/cm³, a % V_(v) of 12%, and a compression set of 0.30 mm (12 mils).V_(o) was 0.023 cm³ /g, V_(ml) was 0.04 cm³ /g (rounded from 0.0433),and V_(mh) was 0.09 cm³ /g (rounded from 0.0889). For Control 1, V_(a)was 0.04 cm³ /g and the ratio V_(a) /V_(ml) was 2.05.

(2) The procedure for preparing "Standard Pressboard" as described inExample 1, Part A, was repeated, except that the plants of the presswere heated to 200° C. and, after loading the press at contact pressure,the pressure was raised to and maintained at 60 kg/cm² (5880 kPa; 850psi) for one hour while the platens of the press were maintained at 200°C. The product, designated as "Control 2", had a high value ofcompression set of 1.0 mm (40 mils). It had a density of 1.07 g/cm³, a%V_(v) of 22%, and an oil absorption of 9.59%. V_(o) was 0.11 cm³ /g,V_(ml) was 0.17 cm³ /g (rounded from 0.1733 cm³ /g), and V_(mh) was also0.17 cm³ /g (rounded from 0.1723). For Control 2, V_(a) was 0.17 cm³ /gand the ratio V_(a) /V_(ml) was 1.0.

(3A) Filaments of MPD-I were prepared substantially as described byGross in U.S. Pat. No. 3,756,908, Column 6, lines 11-23. The resultinghigh modulus filaments were then cut to a floc having a length of about0.64 cm (0.25 in) and then slurried in water to a concentration of about0.3%.

Fibrids of MPD-I were prepared substantially as described in column 5,lines 34-57 of the same patent. The refined fibrids were then dilutedfurther in water to a concentration of about 0.5%, and passed to amixing "T" along with the above mentioned slurry of high modulus floc,at a ratio of fibrid to floc of about 1.55 to 1.0 (60% fibrids and 40%floc). The mixture was directed to the headbox of a Fourdrinierpaper-making machine and then to a forming wire for production of a wetsheet. The wet sheet was then removed from the wire and passed throughsteam heated dryer caps to reduce the moisture content of the sheet toabout 5% or less. The paper was then wound on a roll for furtherprocessing.

The paper was removed from its roll, cut into 20-cm (8-in) squares, andthen platen pressed to produce samples of 2-ply paper substantially asdescribed in column 7, lines 6-11, of the same patent. The samples of2-ply paper were pressed at 70.3 kg/cm² (689.5 kPa; 1000 psi) and 280°C. for one minute. The resulting paper, designated as "Control 3A" had athickness of about 0.25 mm (10 mils), a density of about 0.87 g/cm³, anda % V_(v) of 37% by volume of the paper. V_(ml) was 0.28 cm³ /g (roundedfrom 0.2842) and V_(mh) was 0.18 cm³ /g (rounded from 0.1818). Thecompression set was 1.0 mm (40 mils), the oil absorption was 35.3% byweight of paper, and V_(o) was 0.41 cm³ /g. For Control 3A, V_(a) was0.41 cm³ /g and the ratio V_(a) /V_(ml) was 1.5.

(3B) Filaments of MPD-I were prepared substantially as described byGross in U.S. Pat. No. 3,756,908, column 5, lines 68-75, and column 6,lines 1-7, resulting in low modulus filaments which were then cut to afloc having a length of about 0.64 cm (0.25 in) and slurried in water toa concentration of 0.2%.

Fibrids of MPD-I were prepared as described above for Control 3A andpapers were prepared by combining the fibrid and the low modulus floc ata ratio of fibrid to floc of 1.5 to 1.0 (60% fibrid and 40% floc) in awet 20-cm (8-in) square handsheet mold (e.g., of the type made by Nobleand Wood). Papers made in this way are considered to be essentially thesame as papers made on a Fourdrinier paper machine. The wet sheets wereremoved from the 100 mesh screen of the handsheet mold and dried on hotsheet dryers to reduce the moisture content to about 5% or less. Thesheets were then platen pressed to produce samples of 2-ply paper. Theywere pressed at 70.3 kg/cm² (689.5 kPa; 1000 psi) and 260° C. for oneminute.

The resulting paper, designated as "Control 3B" had a thickness of about0.29 mm (11 mils), a density of about 0.77 g/cm³, and a % V_(v) of 44%by volume of the paper. V_(ml) was 0.58 cm³ /g (rounded from 0.5787) andV_(mh) was 0.38 cm³ /g (rounded from 0.3793). The compression set was1.4 mm (54 mils), the oil absorption was 49.9% by weight of the paper,and V_(o) was 0.58 cm³ /g. For Control 3B, V_(a) was 0.58 cm³ /g and theratio V_(a) /V_(ml) was 1.0.

(4) The procedure of Example 5 for preparing Sample 5D was repeated,using a mixture of 95% fibrids and 5% floc, except that the low densitypressboard was prepared by applying a pressure of 35 kg/cm² (3450 kPa;500 psi) for a period of 45 minutes at 140° C. The low densitypressboard was approximately 1.7 mm (68 mils) thick and had a density of1.00 g/cm³. % V_(v) was 28%. The low density pressboard was dried at120° C. for 4 hrs and pressed at 280° C., under low pressure at firstwith brief cycles of release and reapplication of pressure, then for 5min at 8.8 kg/cm². The product, designated "Control 4", was an aramidpressboard approximately 1.6 mm (62 mils) thick, had a density of 1.12g/cm³, a compression set of 0.14 mm (5.5 mils), an oil absorption of1.4% by weight of pressboard, and V_(o) was 0.02 cm³ /g. % V_(v) was19%. V_(ml) was 0.01 cm³ /g (rounded from 0.0141) and V_(mh) was 0.02cm³ /g (rounded from 0.0173). For "Control 4", Va was 0.02 cm³ /g andthe ratio V_(a) /V_(ml) was 1.2.

The properties and void parameters for all of the pressboard samplesprepared as described in the examples, together with the controlsamples, are listed in the Table. The "Standard Pressboard" (abbreviatedStd. Pressboard) sample of Part A of Example 1, is also listed. In theTable, the samples are listed in descending order according to theircalculated void volume, % V_(v).

                                      TABLE                                       __________________________________________________________________________    PRESSBOARD PROPERTIES AND VOID PARAMETERS                                                                                 Ratio**                            IdentificationSample                                                                  % V.sub.v                                                                         g/cm.sup.3Density,                                                                 wt. %Oil Abs.,                                                                     mmComp. Set,                                                                        cm.sup.3 /gV.sub.ml,                                                              cm.sup.3 /gV.sub.mh,                                                              cm.sup.3 /gV.sub.o,                                                               cm.sup.3 /gV.sub.a,                                                               ##STR2##                         __________________________________________________________________________    Control 3B                                                                            44  .77  49.9 1.4   .58 .38 .58 .58 1.0                               Std. Pressboard                                                                       41  .82  32.5 2.1   .38 .42 .38 .42 1.1                               Control 3A                                                                            37  .87  35.3 1.0   .28 .18 .41 .41 1.5                               *Sample 5B                                                                            26  1.02 17.1 0.15  .17 .27 .20 .27 1.6                               *Sample 5C                                                                            25  1.03 12.1 0.35  .16 .23 .14 .23 1.5                               *Sample 4A                                                                            25  1.04 15.9 0.20  .15 .17 .19 .19 1.2                               *Sample 5A                                                                            25  1.04 13.7 0.21  .10 .10 .16 .16 1.6                               *Sample 5D                                                                            23  1.06 11.7 0.34  .05 .08 .14 .14 3.0                               *Sample 1B                                                                            22  1.08 12.2 0.36  .16 .17 .14 .17 1.1                               *Sample 6A                                                                            22  1.10 10.8 0.27  .08 .12 .13 .13 1.6                               Control 2                                                                             22  1.07 9.6  1.0   .17 .17 .11 .17 1.0                               *Sample 1A                                                                            20  1.11 12.7 0.30  .15 .17 .15 .17 1.1                               *Sample 6B                                                                            20  1.15 8.6  0.18  .06 .14 .10 .14 2.5                               Control 4                                                                             19  1.12 1.4  0.14  .01 .02 .02 .02 1.2                               *Sample 2                                                                             19  1.12 9.3  0.13  .09 .17 .11 .17 1.8                               *Sample 3                                                                             18  1.13 9.3  0.13  .06 .14 .11 .14 2.5                               *Sample 4B                                                                            17  1.15 9.7  0.15  .05 .15 .11 .15 3.0                               Control 1                                                                             12  1.21 2.0  0.30  .04 .09 .02 .09 2.1                               __________________________________________________________________________     *Samples illustrative of the scope of the invention                           **Ratio calculated before rounding V.sub.a and V.sub.ml to two decimal        places                                                                   

What is claimed is:
 1. High density pressboard comprised of 20-95% byweight aromatic polyamide fibrids and 80-5% by weight high temperatureresistant floc, said pressboard having a calculated void volume of 13 to28% by volume of the pressboard, a thickness of 0.5 to 50 mm, a mercuryintrusion volume at low surface/volume, V_(ml), of less than 0.20 cm³/g; a mercury intrusion volume at high surface/volume, V_(mh), of 0.08to 0.28 cm³ /g, an oil absorption by volume in cm³ /g, V_(o), of 0.09 to0.28 and by weight of 8-24 wt.%; and a total available absorption volumein cm³ /g, V_(a), equal to the largest of the values for V_(ml), V_(mh),and V_(o) ; the ratio of V_(a) to V_(ml) being at least 1.1; saidpressboard having a compression set of greater than 0.12 mm but lessthan 0.35 mm.
 2. Pressboard of claim 1 wherein the high temperatureresistant floc is an aromatic polyamide floc and the pressboard has adensity of 1.0 to 1.20 g/cm³.
 3. Pressboard of claim 2 wherein at leasta portion of the floc consists of poly(p-phenylene terephthalamide). 4.Pressboard of claim 1 wherein at least a portion of the floc is glassfiber floc.
 5. Pressboard of claim 2 wherein the aromatic polyamidefibrids and floc consist essentially of poly(m-phenyleneisophthalamide).
 6. Pressboard of claim 5 wherein the pressboard iscomprised of 50-70% by weight fibrids and 50-30% by weight floc. 7.Pressboard of claim 6 wherein the density is 1.02 to 1.17 g/cm³. 8.Pressboard of claim 7 wherein the density is 1.10 to 1.15 g/cm³. 9.Pressboard of claim 6 wherein the compression set is greater than 0.20mm but less than 0.30 mm.
 10. Process for preparing the high densitypressboard of any one of claims 1-9 whereby an aqueous slurry having 0.1to 2% by weight total solids comprised of 20-95% by weight fibrids of anaromatic polyamide and 80-5% by weight of high temperature resistantfloc having a length of 2 to 12 mm, said aromatic polyamide fibrids andsaid high temperature resistant floc having a melting point higher than320° C., the slurry is formed into a waterleaf having a water content of50-95% by weight of the waterleaf; the waterleaf is combined intomultiple layers to form a wet lap; the wet lap is pressed at 100° to200° C. under a pressure of 10 to 60 kg/cm² to form a low densitypressboard having a calculated void volume of 30 to 60% by volume of thepressboard, the low density pressboard is dried, ultimately at 270° to320° C. until substantially no further moisture is evolved and thenpressed at 8 to 350 kg/cm² at 270° to 320° C.
 11. The process of claim10 wherein the high temperature resistant floc is comprised of anaromatic polyamide.
 12. The process of claim 11 wherein the pressboardis comprised of 50-70% by weight of poly(m-phenylene isophthalamide)fibrids and 30-50% by weight of poly(m-phenylene isophthalamide) floc.13. Process of claim 12 wherein the low density pressboard is dried,ultimately at 275°-300° C., and pressed at 275°-285° C. and 15 to 70kg/cm².
 14. The process of claim 13 wherein the final pressboard iscooled under restraint.